The present invention relates generally to high resolution, high rate x-ray spectrometers and more particularly to systems for processing the pulses which are generated in response to detected radiation. The United States Government has rights in this invention pursuant to Contract No. DE-ACO3-76SF-00098 between the U.S. Department of Energy and the University of California.
The pulse processing system of the present invention was designed in response to the need for an x-ray spectrometer capable of use in plasma diagnostics in a fusion reactor. High temperature plasmas produced in such reactors emit considerable black body radiation in the x-ray energy level up to several tens of kilovolts. Measurement of this flux is important in determining the plasma temperatures and in detecting the presence of impurities which produce characteristic x-ray lines.
In such diagnosis, spatial and temporal variations in the radiant energy must be measured. Since temporal variations during the plasma pulse must be observed on a millisecond time scale, the solid angle of the detectors observing the plasma must be large enough to give counting rates adequate for statistically meaningful spectra to be accumulated on a time scale of a few milliseconds.
The combination of very high counting rates and the excellent energy resolution requirement for the observation and separation of impurity spectral lines presents very difficult design problems. The final usefulness of the spectrometer requires that a high rate of analyzable pulses be passed by the signal processing system. Rejection of detected events must be minimized and a high throughput must be realized.
As a further complication, since the whole purpose of a fusion reactor is to produce thermonuclear reactions, the spectrometer must cope with significant production of 14 MeV neutrons. The estimated maximum flux of such neutrons through the spectrometer detectors is in the order of 5000/second. Such neutrons will interact by colliding with the silicon or germanium nuclei in the semiconductor detectors, with resulting signals ranging up to a few MeV. The signal processing system must be able to handle the large signals and recover very quickly to process the x-ray signals in the energy range from 1 keV to 50 keV.
Typically, the semiconductor detectors will detect single radiation events (photons or charged particles) and produce impulses of current into a preamplifier which will output detected signals as step waveforms. These signals will then be processed by pulse shaping circuits to provide pulses having an optimal signal-to-noise ratio and having amplitudes which are linearly relates to the energy absorbed by the detectors due to the detected events. To cope with high counting rates, the total width of the pulses must be minimized.
In the environment for which the present invention is intended, the very short pulse widths involved make series (delta) noise, i.e. the "shot" noise of the input amplifier, the dominant noise in the pulse shaping system.
It is known that series (delta) noise is dependent on the rate of change of the step response in the shaping system. It is also known that a symmetrical triangular pulse will have a minimum and constant slope for a given amplitude and will thus have an optimum shape for increasing the signal-to-noise ratio. Any other pulse shape, such as the Gaussian shape commonly produced in existing pulse shaping circuits will have a poorer signal-to-noise ratio since a pulse with a lower slope along some portion of its length must have a higher (and noiser) slope along another portion if it is to reach to the same maximum amplitude in the allotted time.
Although it has been known that symmetrical triangle pulse shaping would be beneficial in reducing series (delta) noise, suitable apparatus for producing such pulses of the character necessary for use in the present invention has not been heretofore devised.
Symmetrical triangle pulse generation has been achieved by proper integration of a symmetrical biphase delay line pulse, but such delay lines are bulky, are not easily varied in their time scale, and cause severe sensitivity of gain to temperature variations. As a consequence, they are not practical circuit elements for the main channel of a pulse processing system.
All modern spectrometer pulse processors use a method to prevent the analysis of pulses whose amplitude is subject to interference by other signals in close time proximity. Generally speaking, this function is achieved by a "pile-up rejector" containing four elements: (a) a gate at the output of the main pulse processing channel; (b) a parallel fast inspection channel where a short duration signal is generated in response to each detected event; (c) a fast discriminator which produces a logic signal having an output width corresponding to the time that the fast inspection channel pulses exceed the fast discriminator threshold level; and (d) a pile-up detector which examines the fast discriminator logic signals, and, by measuring the time between these signals, senses whether two main pulses may distort the signal amplitude of each other--if not, both of the main pulses are gated through to an output; if so, one or both pulses are not gated through.
The accuracy of the pulse processor will depend to a large extent on the "resolving time" of the system, i.e. on the width of the logic signal outputted by the fast discriminator, which width is determined by the length of time that the fast channel pulses exceed the fast discriminatory threshold level. If two detected events occur within this resolving time, the fast inspection channel cannot recognize them as separate signals and both main channel pulses will be gated through together to the output. This will result in a number of output pulses whose amplitude is the sum of two (or more) separate signals, causing the output spectrum to contain "sum" peaks. In the present environment, these sum peaks will distort the thermal black body spectrum seen from hot plasma discharges because some counts that should appear in the intense low energy part of the spectrum will be shifted into the weak high energy part of the spectrum. Such distortion of the black body radiation will affect the measurement of the plasma temperature.
If the resolving time were constant and reasonably well known, an approximate correction could be applied to the continuum spectrum to compensate for this type of pulse pileup. Unfortunately, existing spectrometers fall far short of meeting the criteria of providing a well determined and consistent resolving time. Typically the fast channel pulses are produced by integration or with a single delay line or simple RC pulse shaping. As a consequence there will be a long exponential tail on the back edge of the fast channel signal waveform, causing the resolving time to be very dependent on pulse amplitude. Such shapes of existing fast channel pulses are thus not desirable in environments where a wide and unknown dynamic range of pulse amplitudes is to be measured.
In addition, most existing systems are disadvantageous in that they provide fast channel pulse shaping having a poor signal-to-noise ratio, requiring the fast discriminator level to be set at a high level in order to reduce noise triggering.
In order to achieve maximum throughput, the system should operate to gate all main channel pulses through which do not actually distort each other. At times a second main channel pulse will begin during the time that the preceding main channel pulse is decreasing from its peak, with the peak of the second pulse occurring after the first pulse has ended. Desirably the peaks of both of these main channel pulses should be gated through to the output since neither peak is affected by the other. However, existing systems do not permit this, generally because of the fact that the main channel pulse stretcher waits until the tail of the first pulse reaches a low threshold level before permitting the stretching of a normal pulse.